While I am NOT a professional automotive painter, I have sprayed many, many different cars, motorcycles, model airplanes, machinery, parts, etc. with many different styles, models, of guns, both cheap and very high quality guns, for a LOT of years.
Please, before you do any sanding, clean the item with Ajax or Comet powder cleaner first, mixed with clean water. If you begin by sanding, you will sand any present silicone on the surface into the paint. After cleaning with Ajax or Comet, then clean with wax and grease remover, using paper towels wetted with wax and grease remover, immediately drying with new, clean paper towels. Then you can sand without trapping contaminants.
Harbor Freight guns WILL give a pretty good finish, if you thin your paint a little more than the paint manufacturers call for. Test your gun adjustment on cardboard before you actually spray for the amount of coverage you are happy with. Always spray first coat very lightly.
HF guns are not high priced, nor are they high quality, but they do spray pretty good for the first few times, but this doesn't last.